Optical waveguide modulators are well known in the art and are used in a variety of applications. For high bandwidth application, when intensity modulation at modulation rates in megahertz range and beyond is desired, the modulators are based on electro-optical materials incorporating voltage-controlled waveguides forming a Mach-Zehnder (MZ) interferometer structure, which enables to convert phase modulation of light propagating in the waveguides into an optical power modulation of a combined light at the output of the Mach-Zehnder structure. Such modulators are conventionally referred to as Mach-Zehnder (MZ) modulators. Many applications also require that the modulated light at the output of the MZ modulator is monitored, and both the amplitude of the modulation and its phase is accurately tracked. For that purpose, in-line optical taps may be used at the modulator output to tap off some of the modulated light to a monitoring photodetector. However, the accuracy with which conventional optical taps can track the time-variant optical signal at the output of the MZ modulators may be insufficient for some applications.